Continuous process for manufacturing superabsorbent polymer

Details Continuous process for manufacturing superabsorbent polymer Continuous process for manufacturing superabsorbent polymer

1998-11-24

2001-02-13

Foelak, Morton (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C521S065000

Reexamination Certificate

active

06187828

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention is directed to a continuous process for making water-insoluble, crosslinked, high molecular weight polymers capable of absorbing and retaining large quantities of aqueous fluids (SAP). These polymers are well known in the art by various names such as superabsorbent polymers, hydrogels, hydrocolloids, and water absorbent hydrophilic polymers, etc. For the purpose of this description the term, “SAP” means superabsorbent polymer, collectively referring to such materials.
Exemplary superabsorbent polymers include crosslinked, partially neutralized polyacrylic acid (see U.S. Pat. No. 4,654,039); crosslinked, partially neutralized starch-acrylic acid graft polymer (U.S. Pat. No. 4,076,663); crosslinked, partially neutralized copolymer of isobutylene and maleic anhydride (U.S. Pat. No. 4,389,513); saponification product of vinyl acetate-acrylic acid copolymer (U.S. Pat. No. 4,124,748); hydrolyzate of acrylamide polymer or acrylamide copolymer (U.S. Pat. No. 3,959,569); or hydrolyzate of an acrylonitrile copolymer (U.S. Pat. No. 3,935,099). The teachings of the above patents as embodiments of superabsorbent materials are hereby incorporated by reference.
Superabsorbent polymers find use in many fluid absorption applications with the primary use being in the field of personal care products such as diapers, sanitary napkins, adult incontinent products, absorption pads for medical uses, etc. The largest market for superabsorbent polymers is found in disposable diapers for infants; see e.g. U.S. Pat. Nos. 3,669,103; 3,670,731 or 4,654,039.
Numerous other patents disclose superabsorbent polymers and their uses, such as U.S. Pat. No. 4,076,663; 4,552,938; 4,507,438; 4,5235,098; 4,820,773 and European Patent Application 189,163.
U.S. Pat. No. 4,985,518 discloses a method of preparing a solid water absorbing resin including mixing a monomer solution of (A) acrylic acid neutralized 70-100 mole percent; and (B) a water-miscible to water-soluble polyvinyl monomer in a combined concentration of up to about 50 wt. %; with water to form a mixed monomer solution, and adding a thermal initiator and a redox initiator to the mixed monomer solution to form an initiated mixed monomer solution at a temperature below the decomposition temperature of the thermal initiator. The addition of the redox initiator thereto and polymerization of the monomers causes a rise in the temperature of the initiated mixed monomer solution to a level sufficient to activate the thermal initiator.
U.S. Pat. No. 4,857,610 assigned to Stockhausen, discloses a moving conveyor apparatus for the continuous production of SAP and copolymers of water-soluble monomers particularly acrylic acid and/or methacrylic acid at up to about 40% monomer solids. The trough-like shape of the conveyor belt changes continuously into an extended flat profile during the polymerization process; starting from the side edges and working towards the center of the trough formed by the conveyor belt. The resulting polymer gel strand is released continuously during the transition of the curved trough-like shape of the conveyor belt into the extended, flat form.
U.S. Pat. No. 4,625,001 assigned to Nippon Shokubai discloses a continuous process to make SAP comprising the steps of continuously feeding an aqueous solution of a SAP forming monomer in aqueous solution to effect polymerization into a water-containing cross-linked polymer along with a polymerization initiator. The vessel is provided with a plurality of mutually parallel rotary stirring shafts each fitted with stirring blades. The process includes finely dividing a water-containing gel polymer issuing from the polymerization in progress by the shearing force of stirring blades generated by the rotation of said stirring shafts while allowing the radical aqueous solution polymerization to proceed without interruption, and continuously discharging the resultant finely divided water-containing gel polymer out of said vessel.
Among the known batch processes for making SAP, there is the aqueous solution batch process at relatively low monomer solids (about 30 solids) coupled to drum-type dryers; there is the aqueous solution batch process coupled to a tunnel-type dryer. The aforementioned processes represent a significant capital cost per unit of SAP produced.
A continuous process has been discovered for use with a variety of reaction vessel shapes which avoids polymerization on a moving conveyor, and the need for agitation equipment thereby providing greater simplicity and ease of operation with relatively low capital cost per unit of SAP produced.
SUMMARY OF INVENTION
This invention is directed to a continuous process for manufacturing superabsorbent polymers in a pressurized, unagitated reaction vessel, the vessel is preferably elongated and vertically oriented. The process is characterized in the beginning by feeding a cold mixture of monomer, initiator and water to the vessel which is at least half full of SAP. The vessel is under pressure, and a head pressure at the inlet end is maintained at a level sufficient to advance the SAP toward the discharge outlet. The SAP forming monomer mixture is charged to the inlet of the vessel at a rate that is kept substantially equal to the rate of discharge of SAP through the outlet of the vessel. The process maintains an essentially constant mass of monomer/SAP in the vessel and the residence time is maintained to obtain a desired high level of monomer conversion. The SAP reaction mass advances in a laminar-like flow without agitation and conditions are maintained so as to minimize breakthrough of gas and/or liquid to the outlet where at least 50% of the vessel volume contains the mass of SAP, preferably at least 75% by volume, and most preferably about 90% by volume SAP is maintained during steady state conditions.
The process is also characterized by a positive temperature gradient in the direction of the advancing polymer, rising to a peak temperature nearer to the discharge (tail) end. The pressure in the vessel is maintained to achieve an efficient polymer discharge rate. The advancing polymer moves with gravity and pressure without mechanical assistance or agitation.
Therefore the continuous process of the invention is characterized by the simultaneous feeding at the head-end of an aqueous solution of SAP forming monomer solution, either in un-neutralized or pre-neutralized condition, the monomer(s) being conveyed to the reactor vessel at a relatively low temperature, preferably from 5° C. to 15° C. The head pressure in the gas space of the feed zone above the SAP mass is maintained at a minimum level of greater than or equal to about 20 p.s.i.g., and is adjusted to provide a means to continuously advance the SAP polymer toward the discharge. The monomer feed solution is preferably kept under an intermittent or continuous inert gas purge, such as with nitrogen. The reaction vessel is equipped with a venting means to regulate the internal head pressure. The continuous polymerization process is followed by steps typically involving drying and pulverizing the discharged SAP polymer, as well as optional post-treatment steps specified hereinbelow. The volume percent SAP maintained in the vessel is preferably from 75%-95% of the volume capacity of the vessel. Most preferably, about 90 volume % of SAP is maintained in the vessel during the continuous process.
The preferred process of this invention comprises continuously feeding an aqueous monomer solution comprising at greater than or equal to about 20%, preferably greater than or equal to 28% and, up to 46% SAP forming monomer content, a cross-linking monomer, and initiator(s). The monomer feed solution passes a zone having an inert gas purge, and is initiated as it enters the reaction vessel. As the charged and initiated monomer feed solution advances toward the discharge end, the temperature increases as polymer conversion occurs. The continuous process is therefore preferably conducted under adiabatic conditions. The preferred temperature gradient in the adva

Affiliated with

Also associated with

Rating

Continuous process for manufacturing superabsorbent polymer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Continuous process for manufacturing superabsorbent polymer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Continuous process for manufacturing superabsorbent polymer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2584142